Image filters have never implemented this check, which means that
filters which affect transparent black falsely claim they can compute
their bounds.
Implemented an affectsTransparentBlack() virtual for image
filters, and a similar helper function for color filters.
This will affect the following GMs: imagefiltersscaled
(lighting, perlin noise now filter to clip),
colorfilterimagefilter (new test case), imagefiltersclipped
(perlin noise now filters to clip).
Note: I de-inlined SkPaint::canComputeFastBounds() to avoid adding
a dependency from SkPaint.h to SkImageFilter.h.h. Skia benches show
no impact from this change, but will watch the perf bots carefully.
BUG=4212
Review URL: https://codereview.chromium.org/1296943002
915881fe74